Published by Utah State University Extension and Utah Plant Pest Diagnostic Laboratory PLP-026-21-PR April 2021 Powdery Mildews on Vegetables Nick Volesky, Vegetable IPM Associate • Marion Murray, Extension IPM Specialist Claudia Nischwitz, Extension Plant Pathologist

Table 1. of and their vegetables hosts Quick Facts in Utah • Powdery mildew is widespread in Utah and Broccoli affects many vegetable, fruit, and landscape Cabbage plants. There are several species of powdery cruciferarum Cauliflower mildew fungi, and typically they target just a Kale single host, or only hosts in related plant families. Brussel’s sprouts • Powdery mildew is easily identifiable as white, Chard “powdery” patches on the surface of leaves. Erysiphe polygoni Beet Beans • Utah’s environment is favorable to this disease. Peas It thrives in warm temperatures, humid plant canopies, and poor airflow. Celery • Manage powdery mildew by growing resistant Cucumber varieties, using cultural pest management Melon practices, and applying organic fungicides Golovinomyces cichoracearum such as sulfur or potassium bicarbonate, or Summer squash conventional fungicides such aschlorothalonilor Winter squash a strobilurin. Cucumber Melon Podosphaera xanthii Summer squash owdery mildew is one of the most easily recognized Winter squash fungal plant diseases. It is categorized by spots Onion or patches of white-to-gray powder-like growth P Leveillula taurica Pepper on foliage, stems, or fruit. Roughly 700 species exist that infect grasses, ornamentals, weeds, fruit trees, landscape Tomato trees, shrubs, and vegetables. The closely-related species of fungi that cause powdery mildew are host-specific (Table 1), meaning they cannot survive without the DISEASE CYCLE proper host. Powdery mildew fungi spread in conditions Powdery mildew fungi are considered “obligate of low rainfall and hot temperatures, making Utah’s parasites” meaning that they cannot grow without a climate the perfect environment. living host.

Late in the growing season, the powdery mildew initiates sexual reproduction in some plant species. This sexual phase forms resting structures on foliage and other plant parts that are important for overwintering survival. The resting structures are fruiting bodies (chasmothecia) that contain the overwintering spores and are resistant to low temperatures and drought. With warm humid conditions in spring, chasmothecia release wind-blown spores that cause primary infections on susceptible hosts.

Fig. 1. Conidia (spores) that form from hyphae cause infections throughout the growing season. Fig. 2. Erysiphe pisi on a pea foliage. Fig. 3. Golovinomyces cichoracearum on Fig. 4. Leveillula taurica on pepper. winter squash.

Fig. 5. G. cichoracearum on lettuce. Fig. 6. Podosphaera xanthii on melon. Fig. 7. L. taurica on onion.

Unlike most other fungi, powdery mildew spores can INJURY spread and germinate without the presence of moisture. When a spore lands on a suitable host, it germinates and In general, infected plants have white-to-gray patches hyphae colonize the leaf surface and specialized hyphal with powder-like growth on the upper- or under-side of structures (haustoria) penetrate host cells to absorb leaves. Less frequently, powdery mildew may also occur nutrients. Within 7 to 10 days of this primary infection, the on young stems, buds, flowers, and young fruit. The characteristic mix of white mycelium and spores (conidia) presence of powdery mildew reduces photosynthesis is visible. In some powdery mildew species (like those in and results in reduced fruit yield and low-quality fruit the Leveillula group), the hyphae grow within host tissue. due to reduced sugar levels. Infections on fruit affect marketability by reducing the product’s aesthetics. The massive amount of conidia that form after the primary infection results in multiple secondary infections Heavily infected vegetable crops may also show throughout the growing season. Under conditions of symptoms of leaf curling, chlorosis (yellowing), and warm and dry weather (75–85° F) with high evening possible dieback. Cucurbit powdery mildew affects humidity, rapid spread can lead to an epidemic, fruit via sunscald from lack of canopy cover and by especially for plants grown closely together. Older foliage occasionally infecting cucumber or melon (but not is more susceptible to powdery mildew and will likely be squash). Infected plants tend to have fewer fruits that are the first infected. reduced in size. Quality may also be affected due poor storability.that are reduced in size. Quality may also be affected due poor storability.

UPPDL, 5305 Old Main Hill, Logan UT 84322, extension.usu.edu/pests Page 2 Fig. 8. Erysiphe heraclei on carrot foliage. Fig. 9. on turnips. Fig. 10. L. taurica on tomato foliage.

Fig. 11. P.xanthii on summer squash. Fig. 12. Erysiphe polygoni on bean foliage. Fig. 13. G. cichoracearum on potato.

MONITORING

Early detection of powdery mildew is critical, so it is important to consistently scout plants for disease. Soon after planting, commercial growers should randomly plant parts becomes completely select 5–10 different locations in the field once a week covered with mycelium growth and check plants for powdery mildew. Home gardeners

condia may be able to check all plants. Observe the tops and (spores) are bottoms of older and mature leaves first. Focus on areas dispersed by wind of the field that are densely planted, crowded by weeds, vigorously growing, and shaded. windblown condia cause secondary spread

leaf infection

fungus survives over summer on infected crop debris. Seed is not considered an important source of inoculum

Fig. 14. Simple diagram of the powdery mildew disease cycle.

UPPDL, 5305 Old Main Hill, Logan UT 84322, extension.usu.edu/pests Page 3 Table 2. Examples of Vegetable Varieties Resistant to Powdery MANAGEMENT Mildew Since powdery mildew can diminish yield, it is important to be proactive with management to reduce disease Bean Contender, Provider, Slenderette pressure. Carrot Bolero Beet Kestrel, Red Acre, Solo Cultural Control Adam Gherkin, Amiga, Arkansas Little Leaf, Bristol, • Plant resistant varieties. Several varieties of vegetable Bush Pickle, Calypso, Citadel, Cool Breeze II, crops are labeled with some level of resistance County Fair, Cross Country, Darlington, Dasher II, against powdery mildew (Table 2). Podosphaera Diomede, Diva, Eureka, Fanfare, Fancipak, H-19 xanthii has several races, and varieties labeled as Cucumber Little Leaf, Harmonie, Iznik, Jackson Supreme, resistant may not be resistant to all races. It is currently Jade, Saladmore, Salt and Pepper, Sassy, Seminis, unknown what specific races are present in Utah. Slice More, Socrates, Southern Delight, Suyo Long, • After harvest, remove or plow infected plant material. SV4719CS, Sweet Slice, Talladega, Tasty, Turbo, Powdery mildew overwinters on plant debris and Tyria, Unagi, Unistars, Vlasstar removal can reduce disease occurrence the Afterglow, Ambrosia, Aphrodite, Arava, Astound, following season. Remove and destroy all infected Athena, Atlantis, Avatar, Camino Europa, parts of the plant (leaves, branches, vines, and Cleopatra, Crescent Moon, Da Vinci, Delicious, infected fruit). Do not place infected plant material in Delightful,, Divergent, Duchess, El Gordo, Escorial, composting piles. Compost pile temperatures are not Melon Full Moon, Hannah’s Choice, Infinite Gold, Lilliput, high enough to kill the pathogen. Lillly, Milan, Orange Sherbet, Rockstar, San Juan, Sapomiel, Savor, Shockwave, Sierra Gold, Solstice, • Increase plant spacing. Properly spaced plants will Sugar Cube, Torpedo, Sugar Rush, Sun Jewel, have better air movement, reducing the canopy Verona humidity that can trigger powdery mildew infections. Avalanche, Dakota, Feisty, Green Arrow, Knight, • Morning irrigation will allow excess moisture to dry Maestro, Mr. Big, Oregon Giant, Oregon Sugar Peas from the ground and leaves, reducing evening Pod II, Penelope, Petite Snap—Greens, PLS 141, humidity. Where possible, switch to drip irrigation. PLS 595, Super Sugar Snap Benchmark, Big Doris, Blanco, Blaze, Capital, Cargo PMR, Charisma PMR, Cinnamon Girl PMR, Cougar, Denali, Apogee, Dynasty, Early Giant, Early Giant, Early King, Edison, Eureka, Everest, Pumpkins Gemini, Gold Medal, Grizzly Bear, Gumdrop PMR, Jill-Be-Little, Magnum, Mellow Yellow, Mustang, Orion, Pegasus, Pipsqueak PMR, Porcelain Doll, Renegade PMR, Ritz, Rival PMR, Shiver, Spark, Sunlight PMR Golden Girl, Sebring Premium, Sligo, Golden Glory, Mabel Dunja, Green Machine, Spineless Summer Squash Perfection, Yellowfin, Golden Glory, Cue Ball, Desert, Gold Star Autumn Delight, Autumn Frost, Autumn’s Choice, Black Bellota, Bon-Bon, Bugle, Bush Delicata, Butterbaby, Butterscotch PMR, Celebration, Winter Squash Golden Butta Bowl, Honey Bear, JWS 6823 PMR, Metro PRM, Narragansette, Speckled Pup, Starry Night PMR, Sun Spot, Table Treat, Taybelle PM, Tiptop PMR, Unique, Waldo PMR Tomato Climstar, Frederik, Geronimo, Granadero, Rebelski Pepper Carlomagno, Pizarro Spinach Mouflon

Fig. 15. Vegetable production with dripline irrigation and adequate spacing can help prevent powdery mildew and other diseases UPPDL, 5305 Old Main Hill, Logan UT 84322, extension.usu.edu/pests Page 4 Chemical Control Note that some products and product combinations can cause phytotoxicity (plant injury), such as: Many fungicides (both organic and synthetic) are • Oils applied at temperature over 85° available for commercial operations and home gardeners. Consider fungicide treatments when the first • Sulfur applied at temperature over 80° F sign of powdery mildew is found. Repeated applications • Mixing oil and sulfur or applying within 2 weeks of should occur every 7–10 days (or according to label each other recommendations).

Table 3. Examples of Fungicides Registered for Commercial Use on Vegetable Powdery Mildew in Utah

Active Ingredient Brand Names Cucurbits Root Crops Leafy Greens Solanaceae Crops Copper sulfate Cuprofix Ultra X X X X Copper hydroxide + mancozeb Mankocide X (see label) X X CosavetO, Microthiol DisperssO, Micro SulfO, Sulfur X X X X ThioluxO Chlorothalonil Bravo, Echo, Equus X (see label) X Benzovindiflupyr + difenoconazole Aprovia Top (see label) Propiconazole AmTide Propiconazole 41.8% EC X Cyprodinil + fludixonil Switch X X X X Azoxystrobin Quadris X X Bacillus mycoides LifeGardOB (see label) Serenade ASOOB X Reynoutrai sachalinensis extract Regalia CGOB X Phosphorous acid salts OxiPhos, Fospite B X X X X Hydrogen peroxide + peroxyacetic acid OxiDate 2.0 X X X X Neem oil EcoWorksB X X X X PureSpray GREENO, Ultra-Pure Oil Mineral oil X Horticultural InsecticideO, SuffOil-XO, TriTekO Soybean oil Bionatrol-MO X X X X Potassium bicarbonate KaligreenO, MilStop SPO X (see label) (see label) X

Table 4. Examples of Fungicides Registered for Home Use on Vegetable Powdery Mildew in Utah

Active Ingredient Brand Names Cucurbits Root Crops Leafy Greens Solanaceae Crops Natural Guard Copper Soap, Monterey Copper X X X X Lqui-Cop Natria Insect Disease & Mite ControlB, Sulfur + pyrethrins X X X Bonide Tomato & Vegetable 3-in-1 Fertilome Broad Spectrum Landscape & Garden Fungicide, Hi Yield Vegetable, Chlorothalonil X X X Flower Fruit & Ornamental Fungicide, Ortho Max Garden Disease Control Spectracide Immunox Multi-Purpose Myclobutanil X X Fungicide Potassium salts of fatty acid Safer Brand Tomato & Vegetable 3-in-1B (see label) Bacillus subtilis strain QST 713 Serenade Garden Disease ControlOB (see label) Oils: mineral-based Bonide All Seasons Horticultural OilB X (see label) X X Monterey All Natural 3-in-1B, Arborjet Eco- Oils: plant-based (see label) 1 Fruit & Vegetable SprayOB O Organic B Biological

UPPDL, 5305 Old Main Hill, Logan UT 84322, extension.usu.edu/pests Page 5 REFERENCES & FURTHER READING IMAGE CREDITS

Schilder, A. (2014). How to get the most out of your 1 J. Schlessselman, American Pytopathological Society fungicide sprays. Michigan State University Extension. 2, 3, 8, 9, 11 Nick Volesky, Utah State University Bogran, C., Ludwig, S., & Metz, B. (n.d.). Using Oils as 4 Dr Parthasarathy Seethapathy, Tamil Nadu Agricultural Pesticides. Texas A&M University. University, Bugwood.org 5 Gerald Holmes, Strawberry Center, Cal Poly San Luis Cannon, C., Murray, M., Schaible, C., & Beddes, T. (n.d.). Obispo, Bugwood.org Vegetable pests of Utah. Utah State University Extension. 6 Tom Isakeit, Texas A&M University Extension Cannon, C., Murray, M., Alston, D., & Drost, D. (2018). 10 Dollymoon, wikimedia commons 2018 Utah vegetable production and pest management 12 Howard F. Schwartz, Colorado State University, guide. Utah State University Extension. https://extension. Bugwood.org usu.edu/vegetableguide/ 13 Phil Hamm, Pacific Northwest Handbooks 14 Chart referenced from extensioaus.com.au Davis, R. M., Gubler, W. D., & Koike, S. T. (2008, (Illustrations by Kylie Fowler) November). Powdery mildew on vegetables. Pests Notes, Statewide Integrated Pest Management Program, University of California. Flint, M. L. (2018). Pests of the garden and small farm: A growers guide to using less pesticide. University of California, Agriculture and Natural Resources. Grabowski, M. (2018). Powdery Mildew of Cucurbits [Fact sheet]. University of Minnesota Extension. Howell, J. C. (Ed.). (2009). Vegetable management guide (New England Region). New England Extension Systems. Hudelson, B. (2010, February). Powdery mildew (vegetables) [Fact sheet]. University of Wisconsin Extension Keinath, A. P. (2018). Cucurbit downy mildew management for 2018 [Fact sheet]. Clemson Cooperative Extension. Klein, C., & Gilsenan, F. (2016). Vegetable gardening: The complete guide to growing more than 40 popular vegetables in any space. I-5 Press. Lutton, J., & Rosenberger, S. (2015). Cucurbit disease field guide (C. Kurowski & K. Conn, Eds.). Seminis. Olsen, W. M. (2011, January). Powdery mildew [Fact sheet]. University of Arizona Extension.

Precautionary Statement: Utah State University Extension and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed herein.

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